Hmm.. first off thanks for your answer. Let's check the Scriptures together to see if your answer jives with it completely.
2 Co 1:21 And it is God who establishes us with you in Christ, and has anointed us,
22and who has also put his seal on us and given us his Spirit in our hearts as a guarantee.
Not much of a guarantee then. Also please notice this Scripture also says it is "God who establishes us".
And you didn't explain how a person goes back to being the old man? Because I have a real issue grappling my mind around this one here. Cuz my Bible says...
Colossians 3:3
3For you have died, and your life is hidden with Christ in God.
4When Christ who is your life appears, then you also will appear with him in glory.
This is a real conundrum isn't it? Because it says we died and now every part of us is found in Christ, so if we lose our salvation (or if Holy Spirit left us even though He promised to never forsake us) we have nothing to go back to. Cuz the old man is DEAD. And now the new man is found in Christ.